The massive use of synthetic chemical herbicides in modern agriculture has raised public concern over the associated risks to human health and contamination of the environment. In addition, a growing number of herbicide resistant weeds have been observed. There is an ongoing effort to identify and develop herbicides that are active on a wide range of photoautotrophic organisms and that are less hazardous to human health and to the environment.
The photosynthetic apparatus in photoautotrophic organisms including plants, algae and cyanobacteria is a common target for herbicides. This apparatus comprises the light reactions that transform light into chemical energy in the form of ATP and NADPH, and the carbon fixation pathway (Calvin cycle).
Light absorbed by the photosynthetic apparatus is utilized to oxidize and split water into molecular O2 and protons, while the electrons liberated are transferred via an electron transport chain and reduce NADP+ to NADPH. The proton gradient formed across the thylakoid membrane serves as a driving force for ATP formation from ADP and phosphate by the membrane enzyme ATP synthase. Several known herbicides are targeted to various sites in the electron transport chain; for example DCMU (Diuron) or Atrazine inhibit electron transfer between two quinones associated with photosystem II, and Methyl viologen (Paraquat) that accepts electrons from photosystem I, thereby producing reactive oxygen species that destroy the photosynthetic apparatus (Trebst A. 2007. Photosynth Res 92:217-224).
The Calvin cycle may be roughly divided to three stages: (1) Carboxylation of ribulose-1,5-bisphosphate (RuBP), a reaction catalyzed by RuBP carboxylase/oxygenase (Rubisco), resulting in the generation of 3-phosphoglycerate (PGA); (2) PGA reduction to glyceraldehyde-3-phosphate (GA3P). The PGA reduction occurs in two consecutive reactions catalyzed by phosphoglycerate kinase and GA3P dehydrogenase (GAPDH) using ATP and NADPH produced in the light reactions. GA3P is isomerized by triose phosphate isomerase to dihydroxyacetone phosphate (DHAP). These two triose phosphates, GA3P and DHAP, are substrates for the production of all carbohydrates and organic matter thereof; and (3) Recycling of RuBP from triose phosphates in a series of reactions. To the best ability of the inventors to ascertain, no herbicide capable of penetrating the cell membrane and targeting the Calvin cycle has thus far been reported.
Several enzymes including Rubisco and phosphoribulose kinase act exclusively in the Calvin cycle; yet, isozymes similar to most of the Calvin cycle enzymes function also in the glycolytic and gluconeogenic pathways. Although inhibitors of glycolysis and gluconeogenesis that are used as anti metabolic drugs in cancer and infectious diseases have been reported (Pelicano et al., 2006. Oncogene 25:4633-4646), no selective inhibitor of the Calvin cycle that is able to penetrate into cells of photosynthetic organisms has been documented.
Nitroimidazole derivatives, particularly 4 (or 5)-nitroimidazole derivatives have been long reported as useful in the treatment of infections caused by pathogenic protozoa including, for example certain species of amoebae (U.S. Pat. Nos. 3,435,049 and 3,493,582).
Additional nitroimidazole derivatives were reported as effective fungicides and anti-bacterial agents that may be used as pharmaceuticals as well as pesticides. For example, U.S. Pat. No. 4,046,773 discloses carbamoyl-imidazole derivative having pesticide activity, which may be also used as an herbicide.
U.S. Pat. Nos. 5,206,257 and 5,380,865 discloses 2-phenylimidazole derivatives that can be used for the control of arthropod, plant nematode, helminth or protozoan.
U.S. Pat. No. 4,235,995 discloses the production of 1,4-disubstituted-3-nitropyrazoles having antimicrobial, parasiticidal, and herbicidal activity. Preferred compounds are 1-alkyl or -alkenyl-4-pyrazolecarboxamides are carbonitriles. The new compounds are particularly useful for the control of bacterial animal diseases.
Japanese Patent Application Publication No. 61109701 discloses herbicide containing 2-methyl-4(5)-trifluoromethyl-5(4)-nitroimidazole as an active ingredient. The herbicide is effective on a wide range of weeds and may be used by soil treatment, foliar treatment or treatment under flooding condition as pre- or post-emergence treatment.
Edwards et al. (Edwards D I et al. 1974, Z. Pflanzenphysiol. Bd. 71:S424-427) showed that Metronidazole (1-β-hydroxyethyl-2-methyl-5-nitroimidazole), known as an antimicrobial drug, is effective in inhibiting the photosynthetic electron transport. Metronidazole was shown to act as a potent inhibitor of ferredoxin-linked NADP+ reduction in a system in which photosystem II is non-operative and ascorbate acts as an electron source. The system included NADP+, broken spinach chloroplasts, intermediate electron carrier (DCPIP) and ferredoxin.
Ornidazole (1-chloro-3-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ol) is known as an anti bacterial and anti protozoan drug. Recently, it has been elucidated that Ornidazole inhibits the activity of the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and triose phosphate isomerase (Pelicano et al., 2006, ibid). Ornidazole is widely used in treating bacterial and protozoa infection in poultry and in mammals, including humans.
There is a recognized ongoing need for effective herbicides with wide range activity that are not hazardous to the environment and are not harmful to mammals.